1. Field of the Invention
This invention relates to a device for monitoring time-temperature histories in which a vapor is allowed to contact a liquid polydiacetylene indicator resulting in a color response, by permeating through a vapor-permeable barrier, whereby a color response occurs only after a definite characteristic induction period.
2. Brief Description of the Prior Art
A host of perishable products such as frozen foods, blood, vaccines and the like require a means for monitoring time-temperature shelf life histories.
Various indicators have been developed in the prior art to meet this need with varying success.
Indicators are known that use a liquid or liquid vapor for activating a color response indicating that a predetermined shelf life has expired. For example, U.S. Pat. No. 3,844,718 discloses a defrost indicator which is activatedby contact of water vapor with a water-soluble ink supported on a hygroscopic substrate.
U.S. Pat. No. 3,768,976 discloses a t-T indicator that depends upon the rate of permeation of gaseous oxygen through a polymer envelope containing an aqueous solution of a red redox dye. Upon oxidation, the red dye turns colorless, indicating that the perishable has been exposed to too high a temperature for too long a time. U.S. Pat. No. 3,615,719 discloses a temperature indicator in which a frozen liquid is separated from an indicating layer by a liquid-soluble barrier. When the frozen liquid thaws, a time delay is introduced by the rate of dissolution of the liquid soluble barrier. Only upon dissolution of this barrier does color indication occur.
U.S. Pat. No. 3,501,297 discloses a mixture of diacetylenes, capable of being converted by contact with warm ethanol vapors to a red color. However, such a color ethanol vapors to a red color. However, such a color change by itself, is insufficient to suggest application as a practical time-temperature history indicator, since the color change may simply indicate that a particular temperature has been exceeded, without an indication either of the length of time that temperature has been exceeded or of the time-averaged exposure at higher temperatures.
U.S. application Ser. No. 911,629 (Patel to Allied Chemical, 1978) described a device for measuring time-temperature histories of an article in which a vapor is allowed to permeate through a vapor-permeable barrier to contact a solid indicator thus producing a color response as a function of the time and temperature. However, the induction periods and total lifetimes at room temperature are generally short, e.g. about one minute to several days and devices with longer extended induction times are needed for monitoring articles having relatively long shelf lives.
U.S. Pat. No. 4,042,336 (1977) describes a device for monitoring time-temperature histories comprising a gas generating means, an indicator means supported on a wick and a rate controlling means for transmission of the generated gas to the indicator means in which it is possible to alter the time span over which the device is functional by incorporating into the wick a quantifier reactive with said gas. However, the device requires a quantifier as an integral part of the indicator system and does not mention or suggest the use of polydiacetylenes as the colorindicating material.